NCERT Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current

NCERT Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current

1. Which of the following correctly describes the magnetic field near a long straight wire?

(a) The field consists of straight lines perpendicular to the wire.

(b) The field consists of straight lines parallel to the wire.

(d) The field consists of concentric circles centered on the wire.

Ans. (d) The field consists of concentric circles centred on the wire.

2. The phenomenon of electromagnetic induction is

(a) the process of charging a body.

(b) the process of generating magnetic field due to a current passing through a coil.

(d) the process of rotating a coil of an electric motor.

Ans. (d) The process of rotating a coil of an electric motor.

3. The device used for producing electric current is called a

(a) generator (h) galvanometer

Ans. (a) generator

4. The essential difference between an AC generator and a DC generator is that

(a) AC generator has an electromagnet while a DC generator has permanent magnet.

(b) DC generator will generate a higher voltage.

(d) AC generator has split rings while the DC generator has a commutator

Ans. (d) AC generator has split rings while the DC generator has a commutator.

5. At the time of short circuit the current in the circuit

(a) reduces substantially. (b) does not change.

Ans. (c) increases heavily.

6. State whether the following statements are true or false.

(a) An electric motor converts mechanical energy into electrical energy.

(b) An electric generator works on the principle of electromagnetic induction.

(d) A wire with a green insulation is usually the live wire of an electric supply.

Ans. (a) False (b) True

7. List two methods of producing magnetic fields.

Ans. Magnetic field can be produced by any of the following methods:

(i) Any magnet-bar magnet, horse-shoe magnet or round magnet can be used.

(ii) A wire carrying current produces a field around it.

(iii) A loop or solenoid carrying current.

8. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a liar magnet? Explain.

Ans. A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid. One end of the solenoid behaves as a magnetic north pole, while the other end behaves as the south pole. The field lines inside the solenoid are in the form of parallel straight lines. By taking a bar-magnet with known north poles near one end of the solenoid and if it shows repulsion then that end of solenoid is north pole and the other end is south pole. The property of magnet i.e. like poles repel and unlike poles attract is used for the determination of poles of solenoid.

9. When is the force experienced by a current carrying conductor placed in a magnetic field largest?

Ans. According to Flemings left hand rule the force experienced by a current carrying conductor placed in a magnetic field is largest when they both are perpendicular to each other.

10. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

Ans. According to Fleming’s left hand rule, the direction of magnetic field is downwards.

11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?

Ans.

Principle: A current carrying conductor when placed at right angle to a magnetic field, experiences a force due to a which we get motion. The direction of the force is given by Fleming’s left hand rule.

Working: Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through Y. The current flows from arm A to B arm and then C to D, the direction of flow of current in both arms is opposite. As per Fleming’s left hand rule, the force acting on arm AB pushes it down while the force acting on CD pushes it upwards. Thus, the coil and the axle rotate anticlockwise. Due to action of split ring commutator at half rotation, split rings P and Q change their contacts with brushes. Now P makes contact with Y and Q with X. As a result, current begins to flow in coil along DCBA, As a result now arm AB is being pushed upward and arm CD downward by the magnetic force. So coil rotate half a turn more in the same direction. This reversing of current direction is repeated at each half rotation and so the coil continues to rotate in the same direction. The split ring helps in changing the direction of the current.

12. Name some devices in which electric motors are used.

Ans. Electric motor is used in all such devices where electric energy is used, converted into mechanical energy to get the motion of machine. E.g. it is used in electric fans, mixer grinders, coolers, A.C., washing machines, computers etc.

13. A coil of insulated copper wire is connected to a galvanometer; What will happen if a bar magnet is (i) pushed into the coil (ii) withdrawn from inside the coil (iii) held stationary inside the coil?

Ans. (i) If bar magnet is pushed into the coil of insulated copper wire, galvanometer shows the deflection as current is induced in the coil.

(ii) When the bar magnet is withdrawn from the coil the galvanometer shows deflection again but now to the opposite side.

(iii) When the bar magnet is held stationary inside the coil, the galvanometer does not show any deflection no induced current.

14. Two circular coils A and B are placed close to each other If the current in the coil A is changed will some current be induced in the coil B? Give reason.

Ans. The current if changed in coil A. Some current will be induced in the coil B.

Reason: When the current in coil A is changed, the magnetic field around it also changes. As the coil B is placed very close to the coil A, the magnetic field lines around B coil also changes due to this as current is induced in it.

15. State the rule to determine the direction of a

(i) magnetic field produced around a straight conductor carrying current.

(ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and

(iii) current induced in a coil due to its rotation in a magnetic field.

Ans. (i) To know the direction of magnetic field produced around a straight conductor-carrying current. Rule used is-Right hand thumb rule’.

(ii) Fleming’s left hand rule is used to find the direction of force experienced by a current carrying straight conductor, when placed in a magnetic field, which is perpendicular to it.

(iii) Fleming’s right hand rule is used to determine the direction of current induced in a coil due to its rotation in a magnetic field.

16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Ans.

Principle: An electric generator works on the principle of electromagnetic induction. When a coil is rotated between the magnet or when the magnet is rotated in and out of the coil the current is induced in the coil and the direction of current is given by Fleming’s right hand rule.

Working: As shown in the figure, when the axle attached to the two rings is rotated such that arm AB moves up and arm CD moves down in the magnetic field produced by the permanent magnet i.e. ABCD rotates clockwise. By Fleming’s right hand rule, the induced currents are set up in these arms and flows in the direction ABCD, which flows from B2 to B.

After half a rotation, arm CD starts moving up and AB moves down. As the directions of the induced currents in both the arms change current is induced in direction DCBA, which further flows from Bl to B2 externally. After every half rotation the polarity of the current in arms changes. Such current which changes direction after equal intervals of time is called an alternating current (AC).

Brushes: are used to transmit current induced externally from coil ABCD to the external circuit.

17. When does an electric short circuit occur?

Ans. When the insulation of a wire gets damaged and this naked wire comes in contact with other such wire i.e. live wire and neutral wire comes in contact, the current flowing in the circuit rises and short circuiting occurs.

18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Ans. The earth wire is connected as a safety measure with all electrical appliances that have metallic body e.g., microwave, electric press, toaster, geyser, cooler, AC, etc. The earth wire provides a low resistance conducting path for electric current. If there is any leakage of current then the user would not get any current because the current flows down into the earth and keeps the potential of the appliance and earth same.